Like a regular business convention, it has panels and discussion and networking as well as freebies and vendors. Unlike a biz con, it's run by volunteers and is designed to be fun as all 2,000+ people hang out and have parties and talk about (and sometimes do) SF, Fantasy, literature, art, science, not science, psychology, sex, and anything else you might imagine. There are costumes, toys, and spaces dedicated to particular things like Harry Potter or Doctor Who. There's a big dance, an art show, and a masquerade where creators of elaborate costumes are judged by experts.

I use one or all of these sentences. Usually this either shuts them up or engenders more specific questions.